1. Field of the Invention
The present invention relates to radio apparatus, and it particularly relates to a radio apparatus using multiple subcarriers.
2. Description of the Related Art
An OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme is one of multicarrier communication schemes that can realize the high-speed data transmission and are robust in the multipath environment. This OFDM modulation scheme has been applied to the wireless standards such as IEEE802.11a/g and HIPERLAN/2. The burst signals in such a wireless LAN are generally transferred via a time-varying channel environment and are also subject to the effect of frequency selective fading. Hence, a receiving apparatus generally carries out the channel estimation dynamically.
In order for the receiving apparatus to carry out the channel estimation, two kinds of known signals are provided within a packet signal. One is the known signal, provided for all carries in the beginning of the packet signal, which is the so-called preamble or training signal. The other one is the known signal, provided for part of carriers in the data area of the packet signal, which is the so-called pilot signal (See Reference (1) in the following Related Art List, for instance).
Related Art List
    (1) Sinem Coleri, Mustafa Ergen, Anuj Puri and Ahmad Bahai, “Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems”, IEEE Transactions on broadcasting, vol. 48, No.3, pp. 223-229, September 2002.
In wireless communications, adaptive array antenna technology is one of the technologies to realize the effective utilization of frequency resources. In adaptive array antenna technology, the directional patterns of antennas are controlled by controlling the amplitude and phase of signals, to be processed, in a plurality of antennas, respectively. One of techniques to realize higher data transmission rates by using such an adaptive array antenna technology is the MIMO (Multiple-Input Multiple-Output) system. In this MIMO system, a transmitting apparatus and a receiving apparatus are each equipped with a plurality of antennas, and packet signals to be transmitted in parallel are set (hereinafter, each of data to be transmitted in parallel in a packet signal is called “stream”). That is, streams up to the maximum number of antennas are set for the communications between the transmitting apparatus and the receiving apparatus so as to improve the data transmission rates.
Moreover, combining this MIMO system with the OFDM modulation scheme results in a higher data transmission rate. For the purpose of enhancing the transmission efficiency in this MIMO system, the data signals to be transmitted respectively in a plurality of packets are aggregated into a single packet. In so doing, the control signals are appended to the respective data signals. In other words, a plurality of combinations of control signals and data signals are contained in the packet signals. It is generally the case that the information amount of control signals is smaller than that of data signals. Here, MIMO is carried out between a plurality of streams to transmit the data signals. On the other hand, the subcarriers to be used respectively by a plurality of streams are so defined as to be varied, and the control signals are divided into streams, respectively, so as to be transmitted.
Under the above circumstances, the weight in the receiving apparatus differs between when the data signal is received and when the control signal is received. In the case where known signals are appended to the header portion of packet signals and the weights are derived, using said known signals, by the receiving apparatus, there is a possibility that the error rate will worsen in the rear combination. Since the control signal contains important information therein, it is required that the control signals be transmitted more reliably than the data signal.